Rotorcraft (e.g., helicopters) and VTOL (vertical takeoff and landing) aircraft have a natural tendency to stir particulates into the air with their downwash when operating near the Earth's surface. This typically occurs while maintaining or transitioning in or out of the hovering flight regime, at an altitude that is within one to two times the equivalent rotor diameter. In certain environments, a relatively high concentration of stirred particulates, typically dust, may significantly obscure the pilot(s) field of view (FOV). The particulates can result in the loss of outside visual references, or brownout (in a dust environment), which can induce spatial disorientation in a pilot. The pilot(s) may not be able to see the ground. Depending on conditions, particulates may comprise dust, sand, snow, or other materials that can become airborne either due to rotor downdraft or local weather conditions.
The difficulty of a brownout situation is further compounded at night, because there are fewer visual cues available. Further, pilots (especially military aviators) may be using night-vision goggles (NVGs) which typically restrict the FOV to 40°. Using NVGs, also referred to as flying “aided,” provides a visual acuity of 20/25 at best [Army 2007A]. Due to the challenges involved in such operations, pilots learn to use a variety of compensatory methods.
One method of compensation is to maintain enough forward airspeed during the approach and touchdown to outrun the formation of particulates and prevent the particulate formation from enveloping the cockpit. While effective in some situations, this method is not suitable for many landing zones, particularly those that are rough, sloping, confined, or pinnacle. Another method is referred to as termination to a point OGE (out of ground effect) [Army 2007B, Army 2013]. This method requires more power. The initial approach is to a high hover position directly over the intended point of landing. The high hover position is used to stir and dissipate the dust before descending to the ground. Hovering OGE is effective is some situations, but there are disadvantages. First, depending on the aircraft gross weight and environmental conditions, the power required may not be available to hover OGE. Second, it may not be a tactically advisable maneuver, because it exposes the aircraft in its most vulnerable state for an extended period. Third, descending from an OGE hover surrounded by a ring of circulating dust can induce spatial disorientation, resulting in improper control manipulation, consequent aircraft drift, and/or an unanticipated, possibly damaging touchdown rate.
Generally, in helicopters, once outside references are lost out of the windshield, focus is directed through the chin bubble and/or other cockpit door windows. If references are lost through the chin bubble and windows, the focus transitions to the flight instruments, and the approach is aborted using an instrument take-off (ITO). Technically, since the maneuver is not initiated from the ground, it is actually more a modified “go-around.” Once above the dust with adequate visibility, the crew may continue visually and re-evaluate the situation.
Regardless of the method employed, there is always the potential to become partially or completely enveloped in dust. At night, when shifting focus from the windshield to the chin bubble or other windows, NVG use provides additional limitations. When looking through NVGs, depth perception is severely limited, especially at close ranges. The resolution at close range may be lower due to individual focus settings or constraints. The pilot may not have a clear sight picture of the immediate ground surface during the final stage of an approach. Cross-checking the chin bubble or cockpit door window looking through NVGs requires a large head movement that can be hazardous during the critical final moments of an approach.
An alternative, not printed in Army training manuals, is used in some cases to provide improved visibility beneath the aircraft. The landing light or search light is turned on, and the pilot looks beneath the NVG eyepieces and through the chin bubble or cockpit door window using the unaided eye. This technique can offer the best combination of available options by allowing the pilot to divide his/her attention by looking through the windshield using the NVGs (arrow 102) at horizon associated references and maintaining a good ground reference cross-check by glancing through the chin bubble unaided (arrow 104), as illustrated in FIG. 1.
One problem with using landing lights or search lights in this way is that tactical considerations may be sacrificed to the intensity of the light. Further, the landing light and searchlight are considered incompatible with NVGs, because they are conventional white lights (unless the searchlight is infrared). The compatibility issue is, however, more of a misconception than a reality with modern NVGs. Modern NVGs, such as the AN/AVS-6(V)3 (Exelis Night Vision, Roanoke Va.), have automatic brightness control (ABC) and bright source protection (BSP) which are built-in features designed to prevent blinding the user or damaging the NVGs. However, most white lights are still not conducive for use with NVGs, because ambient light is amplified approximately 2000-3000 times by the goggles, and BSP has the side effect of lowering resolution [Army 2007A].
Viewing underneath the goggles aided by the landing light or searchlight light works in many instances, but in heavy or severe dust the pilot may still be disoriented due to the intensity. This is likely why the method is not formally recommended. Military helicopters do have infrared searchlights that are considered NVG compatible, but as discussed above, looking through the chin bubble with NVGs requires excessive head movement and provides low resolution viewing, limited FOV, and lack of depth perception. The intensity of the infrared searchlight can also produce disorientation while looking through NVGs in heavy or severe dust just as the landing light can. Although the infrared searchlight has adjustable brightness, it is a very coarse adjustment, has limited directional control, and always defaults to maximum brightness when power to it is cycled.